Virtualization - Jun 2014 - [PATCH 3.4 45/88] virtio-blk: Call del_gendisk() before disable guest kick

3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Asias He <asias at redhat.com>

commit 02e2b124943648fba0a2ccee5c3656a5653e0151 upstream.

del_gendisk() might not return due to failing to remove the
/sys/block/vda/serial sysfs entry when another thread (udev) is
trying to read it.

virtblk_remove()
  vdev->config->reset() : guest will not kick us through interrupt
    del_gendisk()
      device_del()
        kobject_del(): got stuck, sysfs entry ref count non zero

sysfs_open_file(): user space process read /sys/block/vda/serial
   sysfs_get_active() : got sysfs entry ref count
      dev_attr_show()
        virtblk_serial_show()
           blk_execute_rq() : got stuck, interrupt is disabled
                              request cannot be finished

This patch fixes it by calling del_gendisk() before we disable guest's
interrupt so that the request sent in virtblk_serial_show() will be
finished and del_gendisk() will success.

This fixes another race in hot-unplug process.

It is save to call del_gendisk(vblk->disk) before
flush_work(&vblk->config_work) which might access vblk->disk, because
vblk->disk is not freed until put_disk(vblk->disk).

Cc: virtualization at lists.linux-foundation.org
Cc: kvm at vger.kernel.org
Signed-off-by: Asias He <asias at redhat.com>
Acked-by: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: Rusty Russell <rusty at rustcorp.com.au>
Signed-off-by: Ben Hutchings <ben at decadent.org.uk>
Cc: Yijing Wang <wangyijing at huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>

---
 drivers/block/virtio_blk.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -581,13 +581,13 @@ static void __devexit virtblk_remove(str
 	vblk->config_enable = false;
 	mutex_unlock(&vblk->config_lock);
 
+	del_gendisk(vblk->disk);
+
 	/* Stop all the virtqueues. */
 	vdev->config->reset(vdev);
 
 	flush_work(&vblk->config_work);
 
-	del_gendisk(vblk->disk);
-
 	/* Abort requests dispatched to driver. */
 	spin_lock_irqsave(&vblk->lock, flags);
 	while ((vbr = virtqueue_detach_unused_buf(vblk->vq))) {